1. Field of the Invention
The present invention generally relates to optical communications, and in particular, to optical duobinary modulation schemes.
2. Description of Related Art
The ever-increasing demand for telecommunication systems has led to the development of high-bandwidth optical transmission systems having multi-gigabit data rates. As the bandwidth of a telecommunication system increases, previously unimportant system factors become critical to the successful operation of the system.
One factor which has become critical is the chromatic dispersion of an optical data signal, where the broadening of the digital pulses as the signal is propagated through the transmission system degrades the system performance and causes increased bit-error rates. Since the chromatic dispersion is essentially a function of the propagation velocity in a fiber and the wavelength of that light, signals with a broader optical spectrum will be more severely corrupted by inter-symbol interference effects than signals with a narrower spectral content. The broadening of the optical spectral content of a given signal is usually referred to as the xe2x80x9cchirpxe2x80x9d of the signal.
The use of dc-biased narrow-linewidth laser sources in conjunction with zero-chirp external modulators has resulted in the information bandwidth itself being the limiting and controlling factor of the spectral content of a signal, and hence the severity of the inter-symbol interference at a receiver. If all other parameters remain constant for such a system, a reduction in transmission bandwidth should result in a reduction of the chromatic dispersion penalty for the system.
The use of so-called duobinary codes is one way of reducing the information bandwidth, since a duobinary coded signal (i.e., duobinary modulated signal) has half the-transmission bandwidth of a binary signal. A duobinary modulated signal is in fact a ternary signal having three levels: xe2x88x921, 0, 1.
As shown in FIG. 1, in a traditional optical duobinary modulation scheme, first an electrical signal 101a is input to an exclusive-or gate 103. Exclusive-or gate 103 receives input electrical signal 101a as well as a feedback electrical signal 101b. Feedback electrical signal 101b is created by taking an output signal 105 (i.e., output from binary encoder 103), adding a delay to this signal via a delay-block 107, and then providing this signal back to exclusive-or gate 103 as feedback electrical signal 101b. Usually the added delay equals one time slot. Exclusive-or gate 103 and delay block 107 together are known as binary encoder 108.
Exclusive-or gate 103 receives input electrical signal 101a and feedback electrical signal 101b, and creates an encoded binary signal 109. This encoded binary signal 109 is then input to a low-pass filter 111. Low-pass filter 111 receives encoded binary signal 109 and generates a duobinary signal 113. Duobinary signal 113 typically comprises three levels, e.g., 0, 1, 2 or xe2x88x921, 0, 1. The levels of duobinary signal 113 are induced by choosing appropriate bandwidth of low-pass filter 111.
Duobinary signal 113 is then input to an optical modulator 115. Optical modulator 115 receives duobinary signal 113 as well as an optical input 117. Optical input 117 is an optical beam generated by a traditional light source 121, e.g., a laser diode. Optical modulator 115 modulates optical beam 117 with duobinary signal 113 and creates a modulated optical duobinary signal 119. Modulated optical duobinary signal 119 has the same line-rate as the input electrical signal 101a but has optical properties.
FIG. 2 illustrates a timing chart corresponding to the traditional scheme of FIG. 1. The timing chart has been provided to illustrate processing of signals in the traditional scheme. In FIG. 2, graph 201 depicts timing of input electrical signal 101a, and graph 203 depicts timing of feedback electrical signal 101b. As illustrated, electrical signals 101a and 101b have the same line-rate.
Graph 205 depicts timing of encoded binary signal 109, wherein encoded binary signal 109 has been created by inputting electrical signals 101a and 101b to exclusive-or gate 103. Graph 207 depicts timing of duobinary signal 113 wherein duobinary signal 113 has been created by passing encoded binary signal 109 through low-pass filter 111. In graph 207, duobinary signal 113 is shown to have three levels: 0, 1, xe2x88x921.
Graph 209 depicts timing of output optically modulated duobinary signal 119 wherein optically modulated duobinary signal 119 has been created by modulating optical beam 117 with duobinary signal 113.
The modulation scheme described in FIGS. 1 and 2 has many advantages including high fiber dispersion tolerance. However, this scheme requires a binary encoder 108 operating at the line-rate of input electrical signal 101a. As the line-rate increases, for example, 40 Gb/s or higher, it becomes increasingly difficult to implement binary encoder 108 operating at such a high line-rate. For example, there are no commercial binary encoders available in the market that function at a line-rate of 40 Gb/s or higher.
A method and apparatus for generating high-speed modulated optical duobinary signals are provided.
In accordance with the principles of the present invention, two input electrical signals are respectively input to two binary encoders. The binary encoders encode the input electrical signals and generate a pair of encoded binary signals which are then input to an analog amplifier. The analog amplifier amplifies the encoded binary signals and generates a pair of duobinary signals. Then, the duobinary signals are input to a dual-electrode modulator which modulates a pair of optical beams with the duobinary signals to generate a pair of modulated optical duobinary signals. Both modulated optical duobinary signals are then combined to generate the desired high-speed modulated optical duobinary signal.
The principles of the present invention provide a low-cost solution because two low-speed electrical signals may be used as input electrical signals to generate a high-speed modulated duobinary signal having twice the line-rate of the input signals, e.g., if the desired line-rate of the output duobinary signal is 40 Gb/s, the input electrical signals may have a line rate of 20 Gb/s.
Similarly, the principles of the present invention permit the use of low-cost binary encoders. For example, if the desired line-rate of the output duobinary signal is 40 Gb/s, each of the binary encoders of the present invention may operate at only 20 Gb/s (half of the line-rate of the desired high-speed optical duobinary signal). This is a substantial improvement over the prior art, which required binary encoders to operate at the same line-rate as the desired output line-rate.
In one embodiment, the present invention is a method for generating an optical duobinary signal, comprising the steps of (a) converting a first electrical signal into a first encoded binary signal; (b) converting a second electrical signal into a second encoded binary signal; (c) inputting the first and second encoded binary signals to an analog amplifier to create a first duobinary signal and a second duobinary signal; and (d) applying the first duobinary signal and a first optical beam to an first optical modulator, and applying the second duobinary signal and second optical beam to a second optical modulator to generate an output optical duobinary signal.
In another embodiment, the present invention is an optical duobinary signal generator, comprising (a) a first binary encoder configured to generate a first encoded binary signal from a first electrical input signal; (b) a second binary encoder configured to generate a second encoded binary signal from a second electrical input signal; (c) an analog amplifier coupled to the first and second binary encoders and configured to generate first and second duobinary signals from the first and second encoded binary signals; and (d) a pair of optical modulators coupled to the analog amplifier and configured to modulate a pair of optical beams with the first and second duobinary signals to generate an output optical duobinary signal.